1. Field of the Invention
The present invention relates to electroluminescent memory and display arrangements. More paticularly, the present invention relates to electroluminescent devices exhibiting a bistable memory characteristic sufficient to permit electroluminescence to be sustained by an alternating sustain signal.
2. Description of the Prior Art
Solid state electroluminescent materials are considered highly attractive for display device applications and the like. In the main, this is due to the fact that such materials exhibit high efficiencies and relative ease in fabrication. For example, a typical electroluminescent phosphor shows an efficiency of 18-20 lumens/watt. In this regard, even electroluminescent thin films show efficiencies on the order of 1 lumen/watt and there have been reports of possibly 3 lumens/watt. This may be contrasted, for example, to gas panel displays which typically show 0.1 lumens/watt efficiency.
Traditionally, there have been two main problems associated with electroluminescent devices, i.e., they exhibit short life and require a high switching voltage and current. The short life problem has recently been considerably improved. The requirement for a high switching voltage and current is largely a result of the refreshed matrix addressed operation. Typically, utilization of electroluminescent films or layers in a matrix-addressed display operation necessitates the use of high-peak power drivers. Such drivers are generally quite expensive, if (practically) available at all.
Although some effort has been made in the prior art to obtain matrix-addressed electroluminescent storage-type devices with memory, in the main, this effort has resulted in devices which are generally unsatisfactory because of their complexity, cumbersomeness and inefficiency. For example, the U.S. Pat. No. 3,487,378 to Byatt discloses a matrix-addressed arrangement utilizing both electroluminescent and photoemissive materials. However, the Byatt arrangement utilizes a memory function based upon a charge storage mechanism. The Byatt function and operation depends upon a relatively complex structure providing results more elaborate than necessary for a simple matrix-addressed electroluminescent display.
Likewise, U.S. Pat. No. 3,358,185 to Lally and U.S. Pat. No. 3,848,247 to Sherr each describe electroluminescent display arrangements involving the combination of electroluminescent and photoconductive materials and point addressing. However, the devices disclosed by Lally and Sherr exhibit the same disadvantages as the above-mentioned Byatt device. In addition, Lally and Sherr do not provide any basic memory or storage mechanism.
In addition to the above-mentioned electroluminescent display devices, the prior art has also utilized electroluminescent-photoconductive combinations in light amplifier-like devices. One such device has been described by Kazan and Nicoll in an article entitled "Solid State Light Amplifiers," appearing in J. Opt. Soc. Am 47, 887-894, 1957. In this light-amplifier arrangement, a light image projected onto the photoconductive material triggers into an "on" condition the photoconductive-electroluminescent combination, thereby maintaining the image by electroluminescence. However, in such devices, the stability is poor due to the fact that the electroluminescent emission, the ambient light and the photoconductive excitation band are all in the visible range and are overlapping.
A variation in the above-mentioned photoconductive-electroluminescent light amplifier device was subsequently suggested by Kazan in PROC. IRE, 47, 12, 1959. In this latter arrangement, two layers of electroluminescent material are used for light amplification. However, this arrangement, as with previously mentioned electroluminescent memory schemes, suffers from the fact that complicated optical and electrical isolation necessarily had to be utilized in order to achieve bistability. Another bistable electroluminescent arragement typifying the prior art approaches is that described by Janning in U.S. Pat. No. 3,492,549. The Janning bistable electroluminescent arrangement utilizes an insulated gate field effect semiconductor to apply an electric field across the electroluminescent phosphor layer whereby the electroluminescent phosphor layer becomes polarized. The polarized condition of the electroluminescent phosphor layer corresponds to one stable state, while the unpolarized condition of the electroluminescent phosphor layer corresponds to another stable state. In the Janning device, in order to switch to the polarized state, not only is an electric field required to be applied across the electroluminescent phosphor layer but, in addition, a light source is required to simultaneously illuminate the phosphor layer. As can be seen, both the mode of operation and structure of the Janning electroluminescent memory arrangement are optically and electrically complex.
The U.S. Pat. No. 3,432,724 to Frost typifies the matrix-addressed electroluminescent display devices of the prior art which exhibit virtually no memory characteristics, i.e., exhibit no bistability. Not only do electroluminescent display devices of this type suffer from the requirement of high-switching voltage and high-peak current but, in addition, also suffer from scanning and size limitations. As is evident, the scanning requirement, which comes about because of the absence of any memory characteristic, necessitates the use of relatively complex switching circuitry. Typical of the further problems incident the electroluminescent display devices of the Frost variety is the problem of spurious electroluminescent effects or cross effects due to innerelectrode capacitive coupling which causes emission to occur along the row and column selected for generating relatively bright luminescence.